1 83 French National Injiilutf. 



and Le Frari<;^c)is f >uml tlie feconds to be onlv from 6 to 7. 

 It is exceedingly diflu:ulc to avoid tlieft; foiall diifercnccs. 

 Uuiling my oM'c-rvations for three vear?, I tind, by taking- 

 a mean, tliat the appareni. obliqniiv at the fummer foHtice of 

 the year 8 (1800) was 23'' 38' 6"^, or 23'' 27 '58'' for the 

 mean obIi(]nity. Tlie ohfervations of Bradlev , Mayer, La- 

 caille, and Le Genlil, gave nearly 23" 28' 18" for 1750; it 

 would licence refuit that the fecular diminution is only 40'' 

 inHead of the 50'' which arc generdlv fuppofed. A part of the 

 ditforence may aril'e froni the ohfervations, and particularly 

 the old ohfervations, which perhaps ought to be examined 

 with the more correct elements which are employed at 

 prefent. 



Hc/HiZTh OH ihe Fifly Thoufand Sfars, the Ohfervations of 

 wLncb have been publ'ijhed bj Lalande. , , 



Lalande has annonnced, in the preface to his Ilijlo'irc Cf~ 

 Irfle'y tha.t there are in the heavens a great many vacant 

 fpaces, many chansiing (tars, and many red liars. He has 

 now given further details refpefting thele obj'."6ts in a memoir 

 accompanied with tables. By vacant fpaces Lalande here 

 underdands fpaces in whicli no ilars are feen of the ninth 

 magnitude, which are the fniallell that can be perceived with 

 an achromatic tekfcope of iixty-fcven millimetres aperture 

 (about 2I inches) for illuminating the objefl-glafs that the 

 threads niav be feen. It is beyond all'doubt, that, by ex- 

 cluding all foreion iiuht, and employing the ftrongefl magni- 

 fying glaifes, ibele vacant fpaces, properly io called, would 

 be found to be confiderably d\mini(hed in number; and per- 

 haps there is not in the svhole heavens a fingie place to which 

 a telefcope could be pointed without feeing a great number 

 of (la.j, but below tlie ninth magnitude, and confequently 

 too faint to be of any ufe in aftronomy. 



Lalande gives a catalogue of ail thefe vacant fpaces; that 

 is to fav, his table contains the right afcenfion and declina- 

 tion of the centre of each of thefe fpaces. 



The chansiinu; (iars are contained in a fecond table, and 

 are in nunil^er thirty-one. There are only twelve the pe- 

 riods of which are kaown, but there are feveral others which 

 decreafe in magnitude fo as to difappcar at intervals, iiy 

 attentive obfervati<jn, the time which elapies between twa 

 confecntivc difappearances n)ight be determined; and this 

 kind of olifeivation Lalande reconmicnds to the cnrioiity of 

 thofe who, being potfelfed of onlv moderate inftruments, may> 

 Jji.t'.vever, be dcfirous of being ufcful to adrononiv. 



'Ihe third table cQntains thirty-three ftars of a red v^\w\t. 



About 



